1. Field of the Application
This application is directed to multimedia signal processing and, more particularly, to video encoding.
2. Description of the Related Art
Multimedia processing systems, such as video encoders, may encode multimedia data using encoding methods based on international standards such as Moving Picture Experts Group (MPEG)-1, -2 and -4 standards, the International Telecommunication Union (ITU)-T H.263 standard, and the ITU-T H.264 standard and its counterpart, ISO/IEC MPEG-4, Part 10, i.e., Advanced Video Coding (AVC). Such encoding methods generally are directed to compressing the multimedia data for transmission and/or storage. Compression is broadly the process of removing redundancy from the data.
A video signal may be described in terms of a sequence of pictures, which include frames (an entire picture), or fields (e.g., an interlaced video stream comprises fields of alternating odd or even lines of a picture). As used herein, the term “frame” refers to a picture, a frame or a field. Video encoding methods compress video signals by using lossless or lossy compression algorithms to compress each frame. Intra-frame coding (herein referred to as intra-coding) refers to encoding a frame using only that frame. Inter-frame coding (herein referred to as inter-coding) refers to encoding a frame based on other, “reference,” frames. For example, video signals often exhibit temporal redundancy in which frames near each other in the temporal sequence of frames have at least portions that match or at least partially match each other.
Multimedia processors, such as video encoders, may encode a frame by partitioning it into blocks or “macroblocks” of, for example, 16×16 pixels. The encoder may further partition each macroblock into subblocks. Each subblock may further comprise additional subblocks. For example, subblocks of a macroblock may include 16×8 and 8×16 subblocks. Subblocks of the 8×16 subblocks may include 8×8 subblocks, and so forth. As used herein, the term “block” refers to either a macroblock or a subblock.
Encoders take advantage of this temporal redundancy using inter-coding motion compensation based algorithms. Motion compensation algorithms identify portions of a reference frame that at least partially match a block. The block may be shifted in the frame relative to the matching portion of the reference frame. This shift is characterized by a motion vector. Any differences between the block and partially matching portion of the reference frame may be characterized in terms of a residual. The encoder may encode a frame as data that comprises one or more of the motion vectors and residuals for a particular partitioning of the frame. A particular partition of blocks for encoding a frame may be selected by approximately minimizing a cost function that, for example, balances encoding size with distortion of the content resulting from an encoding.
Inter-coding enables more compression efficiency than intra-coding. However, inter-coding can create problems when reference data (e.g., reference frames or reference fields) are lost due to channel errors, etc. In these cases, decoding of inter-coded data may not be possible or may result in undesired errors and error propagation. Decoders may employ concealment schemes which attempt to conceal or cover-up erroneous data with data derived from neighboring blocks or from data in other frames. Improvement of concealment algorithms may provide some improvement in the quality of concealed portions of erroneous video data. However, there is a limit on how high a quality the concealed image will exhibit due to the heavy dependence of concealment algorithms on spatio-temporal information. The concealed data may not be of high quality and the viewing experience may be degraded. In addition, decoding of the signal may become impossible and resynchronization may be required. Error propagation may be limited, and resynchronization (or initial acquisition) may be enabled, by encoding methods that refresh the video. A refreshed video signal can be decoded without reference to or knowledge of other frames.
An independently decodable intra-coded frame is the most common form of frame that enables refreshing of the video signal. The MPEG-x and H.26x standards use what is known as a group of pictures (GOP) which comprises an intra-coded frame (also called an I-frame) and temporally predicted P-frames or bi-directionally predicted B frames that reference the I-frame and/or other P and/or B frames within the GOP. Longer GOP's are desirable for the increased compression, but shorter GOP's allow for quicker acquisition and resynchronization. Increasing the number of I-frames will refresh the video signal more frequently, thereby further limiting error propagation and providing quicker acquisition and resynchronization, but at the expense of lower compression. What is needed is a way to encode video data that limits error propagation of the video stream at the decoder while preserving compression efficiency.